This is not a book for people who don't like the experimental, the boundary-pushing, or the themes which veer close to triggers. The audiobook version features a cast of 166 narrators who aren't identified as they read their parts, out of necessity, and I understand that the print version uses typographical techniques to convey those same changes in point of view. But if the story works for you, it's a thrilling ride that accomplishes things that no other book does, to my knowledge.
Most of the action takes place in a shadowy place occupied by characters, some of them grotesquely altered, who have no idea what is happened to their previous lives and no way to communicate with the people they once knew. Much of the suffering here comes from the way they continue to be stuck in their ways of thinking and being, and fear at what happens when one of them ceases to continue on in this fashion, often after considerable subjective time passing. There is horror at the transition to something worse when a kind of living carapace envelops a person, and little evidence of a place that is much better, yet they do manage to find empathy for one another. In this way, the main characters resolve to find a better fate for the recently arrived boy Willie Lincoln whose mind is still occupied by the family he left and by what he sees happening with his grieving father. By the end, you start to understand what the author has conceived for this afterlife, which does not quite match what the world's religions say about it, and you are challenged to decide what to feel about what has unfolded.
I like to think of the sections set in the "bardo" afterlife in contrast to Dante's Purgatorio, which is mostly a continuous advancement from a lower state to a higher and more perfect state for the souls. There are bestial, agonizing things that go on in the bardo, low comedy, conflict between its inhabitants, and only dimly understood stakes for the characters. These scenes are interspersed with descriptions of the end of Willie's life on Earth and its aftermath, many of them described in a faux scholarly fashion with original sources. Here you can make out the author's idea of what the loss of his son might have done to form the President during the crisis of war, told almost plainly, without fantastic elements. What it amounts to is a work of art strangely fascinating to those readers with enough patience to take it for what it is and pull out a few fragments of meaning.